A re-install sounds like a likely next step.
If that doesn't do the job, you should submit a ticket for technical support from the support page of the website. You can get there through the Support button at the top of this page.

Well i have reinstalled BTP and still the same.If i change the kettle Volume back to us gallon it lets me put any number in, but when i put it to imp gallon i can't change it.is this a fault with BTP.

so all i do is work out what i need in us gallons,put this into my kettle volume tab,then right click on it and change it back to imp gallons.bit strange but this is the only work around i can come up with

brian kay wrote:Well i have reinstalled BTP and still the same.If i change the kettle Volume back to us gallon it lets me put any number in, but when i put it to imp gallon i can't change it.is this a fault with BTP.

It sounds like a bug to me. It would be helpful if you post the issue in the bug reports forum too. This way I won't forget about it

I was finally able to duplicate this.
I had previously only changed the Volume unit setting to Imperial gallons under the Preferences menu, which doesn't seem to change the Kettle Volume and Final Volume to Imperial Gallons. So, I had no problems with changing the numbers.

When I changed the values to "gal (imp)" using option-click (right-click) on the value boxes for Kettle Volume and Final Volume, I was able to reproduce the problem.

I did discover that if you just deleted or selected the number, then changed the number, the error pop-up appeared, but if I triple-clicked to select the number and the "gal (imp)" then typed in the new number, the value was entered without the error pop-up.

I also ran into an additional bug. Sometimes, when you change from"gal" to "gal (imp)", the value will appear so far to the left in the value box that all that is visible in the box is "l (imp)". I can't figure out a pattern to the bug, though.